Solar Activity Effects on Gravity Wave Activity Inferred from Radio Wave Absorption in the Lower Ionosphere

The low frequency (LF) nighttime radio-wave absorption in the lower ionosphere has been measured at Průhonice (50°N, 15°E) in central Europe for over 35 years. Digital measurements, performed since summer 1988, allow absorption oscillations in the period range 10 – 180 mins, which are believed to reflect gravity wave activity, to be derived. Unfortunately, problems with the transmitter in recent years terminated the evaluation of gravity wave activity. The analysis of the available information (∼6 years of data) allows two conclusions to be drawn as to the effects of the solar activity on gravity wave activity: (1) there is no detectable effect of the solar ∼27-day variation on gravity wave activity; (2) there is an indication that the positive effect of the 11-year solar cycle on gravity wave activity in the winter half of the year is remarkable (lack of data in summer). The result concerning the solar cycle effect is, to a certain extent, preliminary, because the available data do not cover a complete solar cycle. A comparison with results from other stations and an interpretation of results are presented.